Coraline needs good lighting to spread. I am not talking about reef lighting but at least power compact would be good. You need to get your alk and calcium up to recommended levels. And you need good flow.
I would maybe get new bulbs for your fixture, hopefully power compact, 50/50 bulbs would be great.
Get a good calcium and alk test kit.
Get some additives to increase calcium and alk if they test low.
Buy a couple more powerheads and add them to your tank.
It will take a little time and patience once you get those things right before you will see the coraline spreading.